Conventionally, a capacitor has been used as a power source of many electronic devices for its good response with respect to charge and discharge. Among capacitors, in particular, electric double layer capacitors have high energy density and are used also for energy storage.
In electric double layer capacitors, due to a spontaneous oxide film formed on a surface of a collector made of a metal foil in air environment, contact resistance on the surface of the collector is increased. Therefore, a configuration in which hard carbon particles are compression-bonded between a collector and a polarizable electrode layer in order to suppress the increase of the contact resistance is disclosed. With this configuration, a distance between the carbon particles entering the spontaneous oxide film and the surface of the collector becomes shorter, allowing a current to pass therethrough. Thus, the contact resistance on the surface of the collector can be reduced.
Note here that an example of conventional art information related to this application includes Patent Document 1.
However, when a capacitor is used for a long time, a compound generated as a result of a reaction between the collector and an electrolytic solution is attached on the surface of the collector that is brought into contact with the electrolytic solution through a gap of a layer of carbon particles. This compound increases the contact resistance (direct current resistance) between an electrode portion and the surface of the collector. This is still a factor causing performance deterioration when a conventional capacitor is used for a long time.
Furthermore, performance deterioration becomes remarkable when the capacitor is used in high-temperature conditions. In the future, when capacitors are used for a vehicle-mounted power source and the like, it is assumed that charge and discharge are carried out in a high temperature car body. Therefore, it is also necessary to enhance the reliability of a capacitor in high-temperature conditions.